The present invention relates generally to optical panels, and more specifically to optical display panels comprising optical waveguides.
When viewing any type of display screen, image contrast is an important parameter which affects viewing quality. To achieve high contrast in all ambient lighting conditions, it is necessary that the viewing screen be as dark as possible. This enables the actual black portions of the image to appear black.
U.S. Pat. No. 5,625,736 discloses an optical display panel defined by laminated waveguides in a wedge form having a narrow inlet face along the bottom thereof, and a vertical outlet screen disposed at an angle thereto. The waveguides are positioned between the inlet face and the screen and are inclined at a small acute angle with respect to the plane of the screen for transmitting image light with total internal reflection within the waveguides.
The image light is transmitted at an angle upwardly through the inclined waveguides, and is therefore not directly emitted perpendicularly to the screen in the typical direction of a viewer. The intensity thereof is correspondingly diminished and reduces the overall brightness of the displayed image. However, the screen may be frosted for diffusing the image light thereat and producing a reasonable viewing image at an increased viewing angle.
U.S. Pat. No. 6,400,876 discloses another type of optical panel wherein horizontally disposed waveguides are stacked vertically in a column with substantially parallel inlet and outlet faces for further reducing the thickness of the panel. A transmissive right angle film is positioned at the inlet face for redirecting image light through the panel. The inlet face and outlet face may be frosted to diffuse the displayed image light.
In both types of optical panels, contrast of the image may be enhanced by introducing blackness in the cladding layers between the cores of the waveguides for absorbing ambient light incident at the display screen. The ends of the cladding layers are exposed at the screen and are directly viewable by a viewer as being black. And, in the '736 patent, a small light acceptance angle at the screen ensures that the ambient light received at the screen is absorbed by the black cladding.
However, scattering or diffusion of the outbound image light for enhancing viewing angle correspondingly decreases perceived contrast or blackness. A conventional diffuser on the display screen diffuses both outbound and incident ambient light, and back-scattering of the ambient light appears milky white at the diffuser which reduces the perceived blackness of the screen.
Accordingly, it is desired to provide an optical display panel having improved diffusion capabilities while maintaining or increasing the panel's perceived blackness or contrast.
These and other advantages of the present invention will become more fully apparent from the detailed description of the invention hereinbelow.